Fix freeze YouTube al cambio punteggio: PNG atomica e relay stabile

rsvg scrive su .tmp poi rename; il feeder ignora PNG parziali.
YouTube relay: no doppio avvio, reconnect RTMP; overlay restart solo dopo 3 miss.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/services/streams/overlay/refresh.rb

@@ -37,10 +37,12 @@ module Streams
       end
 
       def render_png!(svg_path, png_path)
-        unless system("rsvg-convert", "-w", "1280", "-h", "720", "-o", png_path.to_s, svg_path.to_s, out: File::NULL, err: File::NULL)
+        tmp = "#{png_path}.tmp"
+        unless system("rsvg-convert", "-w", "1280", "-h", "720", "-o", tmp, svg_path.to_s, out: File::NULL, err: File::NULL)
           raise Error, "rsvg-convert fallito per session #{@session.id}"
         end
 
+        File.rename(tmp, png_path)
         FileUtils.touch(png_path)
       end
     end